The present invention relates to an electrostatographic copying apparatus comprising means for automatically sensing toner density and adding toner to a developing unit when the sensed toner density drops below a predetermined level.
In such an apparatus, a photoconductive drum or like member is uniformly charged and radiated with a light image of an original document to form an electrostatic image through localized photoconduction. A developing unit applies a developing mixture of toner and carrier particles to the drum to develop the electrostatic image into a toner image.
Whereas the toner is consumed in the developing process, the carrier particles are not. Thus, the toner density, which is defined as the ratio of toner to carrier particles in the developing mixture, progressively decreases. If this process were allowed to continue, the optical density of the copies would progressively decrease to zero.
This effect is overcome as disclosed in Japanese patent publication Nos. 43-16199 and 50-22642 by sensing the density of the toner image after development. The higher the toner density, the higher the sensed density of the toner image. Typically, a non-image portion of the drum which was charged and developed but not imaged will be sensed. As an alternative, a peripheral portion of the drum radiated with a light image of an object having a predetermined optical density is sensed. This compensates for variation in the intensity of a light source which illuminates the document. In accordance with this system, additional toner is added to the developing mixture in the developing unit when the sensed density of the toner image is below a predetermined value.
However, this system as has been proposed thus far in the prior art does not function to accurately sense and adjust the toner density to a predetermined value. This is because the sensed density of the toner image varies in accordance with changes in the charge potential applied to the drum, fatigue of the drum, the intensity of the light image, the developing bias voltage and variations in the power supply voltage and sensor output in addition to the actual toner density.